Mastodon's drummer, Brann Dailor, admits that this style presents a daunting combination. "Metal is scary to some, and 'prog' is scary to others," he says. "So we know we're going to frighten a lot of people - or hopefully, we will."

Not to disappoint Dailor, but lately his band has been attracting fans faster than they've been alienating them. In the last month, Mastodon's latest work, "Crack the Skye," has sold nearly 100,000 copies, and this weekend they'll headline two shows in the city: tomorrow at the Fillmore and Sunday at the Music Hall of Williamsburg.

The Atlanta-based band formed in 1999, bonding on as great an affection for hard-core metal as anything from the post-psychedelic world of art rock. Their full length debut, "Remission," arrived in 2002, but they didn't focus their style until 2004's "Leviathan."

That's the disk based on "Moby Dick." "It just fit in with our lives at the time," Dailor says. "Here we were, a bunch of scumbags, riding around in a van chasing something that may or may not be there. In fact, we found, it's not there."

Despite such bouts of self-deprecation, Dailor and his band remain committed to what he calls "grandiose themes, with music made on an epic scale."

They decided to bring that to the next level on "Crack the Skye," by making it a Russian-themed piece, complete with a four-part suite. Dailor says his interest in Russia can be traced to his girlfriend, who has been in love with the country since she saw "Dr. Zhivago" as a child. Through her, Dailor got into Russian fairy tales and the weird world of Rasputin, who gets name-checked in the lyrics.

Their riskiest move was to slow the music down some from their hard-core roots. "We wanted to step back and create something with a little more space," Dailor says. "Also, Brent would first work on the songs acoustically, sitting around in his underwear at his house in the morning, which isn't exactly conducive to writing fast, crazy metal songs."

To help hone the new sound, the group snagged superproducer Brendan O'Brien, best known for his work with Bruce Springsteen and Pearl Jam. One wonders if the band was at all self-conscious about employing someone so mainstream? "He's not known for bubblegum," Dailor counters. "He's known for making legendary rock bands sound awesome."

Which suited their goal: to make what Dailor calls "the kind of album we could play for our heroes."

"It's supposed to a classic rock album," he says, "or our version of it, anyway."